To solve the problems of gradual depletion of fossil fuels and worsening environmental conditions, and to meet the energy demand for improving the quality of life, studies on alternative renewable resources are under way. Among several renewable resources, only some biomass species which are inexpensive and abundant in nature are sustainable sources of supply; otherwise, most resources are conventionally derived from liquid fuels and fossil resources. The development of a catalytic process for producing various platforms and value-added chemical materials directly from carbohydrates or directly from lignocellulosic biomass requires enormous effort. Among these chemicals, γ-valerolactone (GVL) is recognized as a versatile building block that can be used as an additive in liquid fuels for transportation, as a precursor for the production of polymeric monomers, and a precursor for the synthesis of various value-added chemicals including organic solvents and bio-oxygenates. GVL has proved itself as an excellent green solvent for biomass processing due to its extraordinary physicochemical properties such as low melting and high boiling-point, a remarkably low vapor pressure even at an elevated temperature, a ready miscibility with water without forming an azeotropic mixture, etc.
Generally, three major strategies have been explored for GVL production from levulinic acid (LA) and an ester thereof, based on diversity of hydrogen sources. Molecular H2 is the most common hydrogen source used for this reaction, which occurs in the presence of various metal catalysts (e.g., Ru, Pt, Pd, Ni, Co, and Cu). Formic acid (FA), which forms an equimolar amount with LA during acid hydrolysis of carbohydrates, is also utilized as a hydrogen source to produce GVL from LA to embody the principle of atom economy. Several catalytic systems including nickel promoted copper-silica and Ag—Ni—ZrO2 nanocomposites have been successfully utilized for utilized for quantitative conversion of LA into GVL by consuming equimolar FA as a hydrogen donor. However, the two hydrogenation strategies described above have some limitations (e.g., harsh reaction conditions, use of corrosive acids, and use of precious metals and non-environmental-friendly solvents) thus hindering their application to a large-scale production to some extent.
Recently, Dumesic et al. reported for the first time a catalytic transfer hydrogenation (CTH) method based on the principle of Meerwein-Ponndorf-Verley (MPV) reduction for the hydrogenation of LA and alkyl levulinates to GVL in the presence of a heterogeneous catalyst using secondary alcohols as a hydrogen donor. They have demonstrated that ZrO2 displayed better activity than other metal oxides due to its amphoteric nature. The chemical selectivity of MPV reduction to the carbonyl groups of aldehyde and ketones, replacement of molecular H2 by alcohols, and effective performance of catalysts containing non-precious metals provide a cost-effective alternative for the production of GVL. Accordingly, various catalytic systems, mainly based on zirconium such as ZrO2, ZrO(OH)2, and amorphous Zr-complexes [zirconium 4-hydroxybenzoate (Zr-HBA), zirconium phosphonate], were reported for the above action within a short period of time. Until now, only one report which describes the conversion of LA into GVL by the CTH reaction using crystalline porous material with an appropriate surface area (Zr-beta zeolite, 474 m2/g) is available. However, the complex and time-consuming method for preparing catalysts limits its practical applications.
Generally, an organic/inorganic hybrid nanoporous material is also called “porous coordination polymer” or “metal-organic framework (MOF)”. Metal-organic frameworks consist of metal-nodes or clusters, bridged through organic ligands to form a well-ordered, highly-crystalline porous network having a pore structure of molecular size or nano size (FIGS. 8A-8B). The excellent properties of the metal-organic frameworks such as a large surface area, fine-tunable pore size, coordinatively-unsaturated metal sites (CUSs), functionality of metal ions and organic ligands provide additional advantages in catalysis over non-porous and zeolitic materials. In particular, according to the coordination number and/or kinds of the central metal and the length and/or kinds of ligands, not only particular catalytic reactions can be performed within the metal-organic frameworks of various crystalline structures but also pore size for various reaction environments enabling selection of reactants and/or products and functionality thereof can be provided, and also the conditions for catalytic reactions can be controlled.
As used herein, the term “coordinatively unsaturated metal site” refers to a position as a site for the coordination of metals where a ligand coordinated by the metal ions of a metal-organic framework, representatively water, an organic solvent, etc., are removed, in which another ligand can form a coordination again. The coordinatively unsaturated metal site may be one formed by partial or entire removal of water, solvent molecules other than water, or ligands, which were contained in the metal-organic framework. In order to secure the coordinatively unsaturated metal site of the metal-organic framework, a pretreatment to remove water or solvent components bound to the metal-organic framework may be performed. As a method for the pretreatment, any method that can remove water or solvent components may be used as long as it does not induce a modification of the metal-organic framework. For example, the pretreatment may be achieved by heating to a temperature of 100° C. or above under reduced pressure, or the solvent-removing methods known in the art such as vacuum treatment, solvent exchange, sonication, etc., may be used without limitation.